The present invention generally relates to data transmission systems, and more particularly to a data transmission system capable of transmitting data with a high reliability.
In order to improve the reliability of data transmission, it is possible to provide dual or multiple transmission lines and transmission equipments. However, if the dual transmission lines and transmission equipments were simply used, data dropout inevitably occurs when the transmission line and transmission equipment being used are switched to other transmission line and transmission equipment because no data is transmitted during the switching.
FIG. 1 shows dual ring transmission lines 11 and 12 which couple nodes (stations) A, B, C and D. If the counterclockwise transmission line 11 fails between the nodes B and C, for example, the data transmission from the node A to the node C is made using the clockwise transmission line 12, that is, from the node A to the node C via the node D. A similar rerouting is made when a failure occurs at other parts of the transmission line 11
The dual ring transmission lines of the type shown in FIG. 1 is used in local area networks (LANs) which couple computers and terminals, and the data is transmitted in the form of packets in such LANs. When a failure occurs in the LAN, the cause of the failure is analyzed, and a part of the transmission line where the failure exists is disconnected from the nodes in order to avoid using the transmission line with the failure. For example, a loop-back is made at the node, or the defective transmission line is switched to a normal transmission line.
In a system provided with dual transmission lines, the transmission line which is presently in use may be switched to the other transmission line if a fault occurs in the transmission line in use, thereby making it possible to improve the reliability of the data transmission.
However, a problem occurs when the transmission line in use is switched from one to another. In other words, when a data error occurs due to an abnormality in the transmission line or a deterioration of an element within the node, it takes a certain time to detect the abnormality in the transmission line or the deterioration of the element. For this reason, even when the transmission line is switched immediately after the abnormality is detected, for example, the data is lost from the time when the data error occurs until the transmission line switching is actually made, and the reliability of the data transmission becomes poor due to this switching. The switching of the transmission line effects all nodes which are used for the transmission, regardless of whether or not the node is directly connected to the abnormal transmission line. Therefore, the data dropout effects all nodes which are used for the transmission.